Found in SP/SSW Employee Timetables in the 'Special Instructions' section are (among other things), the Scale Test Cars.

These traveled to places that had Scales (also known as weigh bridges), used to weigh freight cars.

They were used to test and certify that the Scales were accurate, as some freight or lading was charged by weight (usually a bulk commodity), and perhaps also to check that the freight car plus load did not exceed the axle loading of its intended route.

A number of Rules applied to some Scale Test cars.

This is from a 1987 SP Western Region ETT, but is indicative of other Regions and years.
Depending on Region/Year the Sections were sometimes numbered and numbering reflected complexity of the area covered, and the subsections were simply listed.

• Special Instructions
  • Section C SPEED RESTRICTIONS - CARS
    Restrictions must be respected when operating on any foreign railroad except when their requirements are more restrictive.
    • 4. Miscellaneous Cars
      • Scale Test Cars WO-2, SPMW 5868 and SSW 99203..............30 MPH

• Special Instructions
  • Section E TRAIN MAKEUP RESTRICTIONS
    • 13. Scale Test Cars WO-2, SPMW 5868 and SSW 99203 must be handled at rear of train.
      

Over the years there have been many Scale Test Cars on the SP/SSW system. Here is a list of some unit numbers that I have found in ETT's.

• NBS-1
• SPMW 2024
• SPMW 2025
• SPMW 5007
• SPMW 5090
• SPMW 5868
• SPMW 6336
• SSWMW 99203
• WO-2
• WO-3
• WO-4
• WO-5
• WUTX-2
• RGX 450

In a 1979 Western Division ETT, the SPMW cars were allowed 40 MPH on main tracks and 30 MPH on branches.

NBS-1 was allowed 60 MPH but was to be entrained not more than 20 cars ahead of the caboose.

By 1989, Scale Test Cars could be designated as rear-end cars (RE) which meant that they had to be entrained within the rear five cars, but could not be the last car. This was due to the elimination of cabooses.

If you would like or intend to run a Scale Test Car on your layout; then to be accurate, consult an Employee Timetable for the year/era for types present and their operating conditions.

Notes

5007 was listed in a 1979 ETT as a Scale Test Car. I photographed it on a train over Tehachapi in 1998, and it was lettered as a 'Monitor Car' either in the intervening years its purpose had changed or looking at the converted tank cars 6336 and 6338, Monitor and Scale Test are the same thing.

From the very last SP Lines ETT 4/14/1996 only SPMW5868, SSW 99203 and RGX450 are listed and they have a speed limit of 25 MPH and must be entrained within the rear five cars.

Although not listed in the last years of ETT's #2024 and 2025 are still around today (9/28/03) and still listed in UP's computer system. #2024 was seen recently in the Fresno area checking scales; it was confirmed as having a build date of 5/56 weighing 97 tons and a length of 60 feet.

Still listed on UP's books:

• SPMW 2024
• SPMW 2025
• SPMW 5007
• SPMW 5090
• SSW 99203
• RGX 450

SSWMW 99203 (traces as SSW 99203 since SSWMW is longer than the 4 letter standard)

Models:

HO Scale:

Most of the units pictured are of the Flat car variety, if you are not too fussy, then a similar flat car kit could be modified to resemble these. The most distinctive is the two axle units, these were actually made by Walthers in HO Scale and if they are not absolutely accurate, could be made so. The two axle ones are also the most restrictive as far as operations are concerned.